One of the most widely used plastic encapsulations for small-power semiconductor devices, such as transistors, thyristors, or similar three-lead semiconductor devices, is the well-known TO-92 package, which has a shape of a cylinder with a segment cut off parallel to the center line. The semiconductor chip and its leads are hermetically embedded in plastic and, thus, protected against mechanical and chemical influences, such as the penetration of moisture.
In the fabrication of such semiconductor devices with a plastic package, the manufacture of the chip begins with a semiconductor wafer on which several hundred to several thousand identical chips are fabricated. The manufacture of the leads for the chips begins with a lead frame in which several thousand leads are firmly interconnected by cross-links which are removed in one of the last process steps before the final test in order to remove the short circuit caused by the cross-links.
In the set of leads belonging to a single semiconductor device (in the case of the TO-92 package, this is normally a lead triplet), the inner end of one of the leads has such a large area that the chip can be conductively attached thereto, e.g., by means of a conductive adhesive.
During manufacture, the bare lead frame unwinds from a reel and first reaches a chip-mounting station, in which the individual chips are removed from the semiconductor wafer and mounted on the lead area intended for this purpose. At a bonding station, the inner ends of the other chip leads are then connected with contact areas of the chip by fine wires. Thereafter, the chip and the fine bonding wires are covered with a synthetic-resin material, after which the lead frame is reeled again, because packaging is performed in a fabrication step spatially separated from the steps described so far. As a result, the time required by a lead frame to pass through all stages of the manufacturing process is very long.
The mounting of the chip on the lead frame, the bonding of the latter to the contact areas of the chip, and the covering of the chip are spatially separated from the packaging step for several reasons. First, the devices needed for these operations greatly differ in size, so that they cannot be directly spatially associated with one another. Second, the encapsulating devices are presses, for example, which operate with high pressure and, thus require heavier foundations than the chip-mounting, handling and covering devices. Third, the chip-mounting devices and the encapsulating presses have different cycle times, which are difficult to adapt to one another.
The manufacturing method just described also results in so-called webs between the individual leads, which are unavoidable because one of the aforementioned cross-links is used to seal the injection or compression mold. These webs are removed in a separate, sandblasting-like process step, so that the lead frame, which are wound on a reel after the encapsulating step, must be transported to the place where this step is performed.
Finally, the manufacturing process described requires the use of plastics whose composition is adapted to the specific requirements of both the packaging process and the above-mentioned properties, i.e., the selection of the plastics is subject to a number of constraints. For example, they must be sufficiently flowable at as low a pressure as possible so as to fill the mold without forming shrink holes, and nevertheless perform their protective function on the finished semiconductor device.
Although the aforedescribed process is a well-proved mass-production process since it is the least expensive process developed so far and has gained wide acceptance that it can be described as today's standard process, efforts are still being made particularly with a view to further reducing the manufacturing costs of semiconductor devices.
Accordingly, the object of the present invention is to provide a semiconductor device which has a plastic package of an improved design compared to the hitherto widely used plastic packages in order to achieve a manufacturing cost reduction. It is another object to provide a manufacturing technique and apparatus which permits the various steps used to assemble the device to occur in close proximity to one another.
An important feature of the present invention provides for the manufacture of the plastic package, hitherto made on the lead frame, by embedding or molding the chip in a can-shaped housing in a separate step, which offers the advantage of greater freedom in the choice of the starting plastic and the temperatures and pressures to be used in the packaging process. For example, the can is filled with a cast resin prior to the insertion of the chip, etc., and the can placement station can be placed in close proximity to the stations for mounting the chip, etc.
The invention offers the following advantages. A low-cost plastic can be used for the can. The step of removing the webs and, thus, the device necessary for this purpose are eliminated. The presses for producing the can-shaped housings can be simpler in design and, thus, smaller than the presses used so far. The need for two transfers of the equipped lead-frame reels (from a conventional assembly station to the presses and from the presses to the sandblasting station) is eliminated. This reduces the total time of passage of the lead frame considerably, e.g., by more than one half. All process steps necessary to manufacture the plastic package with the chip and its leads can be performed in a single assembly facility which does not take up more area than a conventional assembly facility, so that a large number of such assembly facilities can be accommodated in a manufacturing shop as before, because the can placement station can be located above the stations for assembly.